Question

A thin lens is made with a material having refractive index $\mu =1.5$, both the sides are convex. It is dipped in water $(\mu =1.33)$. It will behave like

• A. a convergent lens
• B. a divergent lens
• C. a rectangular slab
• D. a prism

Answer 1

The correct option is A a convergent lens

Here, we apply the lens maker's formula,
$\frac{1}{f}=(\frac{{\mu }_2}{{\mu }_1}-1)(\frac{1}{R_1}-\frac{1}{R_2})$
Here, ${\mu }_1\ne 1$ (since the lens is not surrounded by air) infact, ${\mu }_1=1.33$
So, $\frac{1}{f}=(\frac{1.5}{1.3}-1)(\frac{1}{R}+\frac{1}{R})$
[Since, $R_1$ is +ve and $R_2$ is -ve for a biconvex lens]
We don't have to find the value of f hence we see that f is +ve.
Thus, it will behave as a convergent lens.